A PLL typically employs a phase detector, loop filter, voltage controlled oscillator (VC)) and feedback loop to generate an output signal from a reference signal. When PLLs are integrated on CMOS integrated circuits with either an on-chip or off-chip loop filter, a fully differential control voltage of the voltage controlled oscillator VCO must be used to avoid noise injection. Otherwise unwanted spurs or degraded phase noise can occur.
Generally, for simplicity, when an off-chip loop filter is used, use is made of a single ended control voltage. While a single-ended control voltage does not degrade performance when an off-chip loop filter is used, when an on chip loop filter is used or when minimized charge pump currents are required this approach cannot be employed.
One possibility is to use a fully differential control voltage approach, as shown in FIG. 1, but this approach is not commonly used since the source and sink current sources used in the charge pump cannot be equal unless an automatic regulation system of common mode voltage control is used because the two current sources in a semiconductor device are never equal due to process variation and other inequalities.
To avoid process variation, switched current sources are commonly used, as shown in FIG. 2. To ensure that the current sources always are saturated, i.e. turned on, two additional switches short the high impedance current sources to a low impedance voltage source that in the single output variant tracks the voltage at the output.